1. Field of the Invention
The invention relates to a system having high-temperature fuel cells with heating of the reaction gases, as generically defined by the preamble to claim 1.
2. Description of the Related Art
High-temperature fuel cells have operating temperatures in the range between 850 and 1000.degree. C. Since only some of the energy of the fuel is converted into electrical energy, heat is created at the anode where the electrochemical process takes place, and this heat must be dissipated to the outside. This heat is predominantly dissipated out of the cell by means of an air stream amount in the cathode, which may be many times larger than that necessary for stoichiometry alone.
The heat of the gases flowing out of the anode and the cathode can be used to bring the gases flowing into the anode and cathode, via heat exchangers, to a temperature required for operation of the fuel cells and tolerable by them. Fuel cells with ceramic structural elements, however, do not tolerate major temperature differences. Yet this is only a problem that affects the peripheral processes in conjunction with fuel cells.
The energy that must be brought to bear in order to bring the air quantity flowing through the cathode to the operating pressure reduces the efficiency, and it is greater the more the air quantity exceeds the stoichiometrically required amount.
From the book entitled "System Design and Optimization in Fuel Cell Systems", edited by L. J. M. J. Blomen and M. N. Mugerwa, Plenum Press, New York 1993, pp. 201-244, it is known to utilize the heat of the air emerging from the cathode to heat the fresh air. This exchange takes place in a heat exchanger.
The exhaust gas with the residual hydrogen contained in it is generally combusted in an afterburner. Depending on the degree of conversion of the hydrogen in the fuel cell, this proportion of hydrogen, in terms of the total hydrogen used, is between 10 and 20%.
In fuel cells in which the fuel, such as natural gas, is converted or reformed with water vapor in the cells themselves, or in other words internally, into CO and H.sub.2, it is necessary, for the sake of reliably avoiding soot emissions, to work with a water vapor excess.
In high-temperature fuel cells, O.sub.2.sup.2- ions pass through the electrolyte from the cathode to the anode, where along with the hydrogen present there, water vapor is created.
If the gas emerging from the anode is diverted--admittedly, while utilizing the great majority of its heat--then prepared water must be evaporated constantly and supplied with the hydrogen to the anode; this is a requirement that in decentralized systems limits the possible uses of the fuel cells and also lowers efficiency.
The heat exchangers employed in the concepts that have become known until now are very expensive in terms of the space they require and their cost, especially heat exchangers for high temperatures.